The invention concerns a method for controlling an internal combustion engine with a common-rail system.
In an internal combustion engine, the quality of combustion and the composition of the exhaust gas are critically determined by the start of injection, the quantity of fuel injected, and the end of injection. In order to stay within legally prescribed limits, the start and end of injection are usually automatically controlled by an electronic control unit. Between the energization of the injector, the needle stroke of the injector, and the actual start of injection, there is a time delay, so that the actual injection start differs from the set injection start. This causes unequal cylinder-specific operating values and exhaust gas values of the internal combustion engine for one and the same operating point. The same applies to the end of injection. Another source of uncertainty is that, in actual practice, the quantity of fuel is not measured directly but rather is computed from other measured quantities.
DE 197 26 756 A1 discloses a method for controlling an internal combustion engine with a common-rail system, in which the rail pressure is detected as a directly measured quantity, and the fuel quantity is computed by a mathematical function, for example a linear or root function, or by an input-output map. According to the information provided in the cited source, the method is supposed to be real-time-capable in that the fuel quantity is directly determined from the current rail pressure. However, the injection rate and the pump delivery rate of the high-pressure pump, for example, are superimposed in a system-specific way on the rail pressure signal, so that the fuel quantity computed in real time contains errors, or the rail pressure must first be filtered, as described in DE 31 18 425 A1.
The method described in DE 197 26 756 A1 is intended for a conventional common-rail system. The method cannot be used directly in a common-rail system with individual accumulators. The common-rail system with individual accumulators differs from a conventional common-rail system in that the fuel to be injected is taken from the individual accumulator. The feed line from the rail to the individual accumulator is designed in such a way in practice that feedback of interfering frequencies into the rail is damped. During the injection interruption, just enough fuel continues to flow from the rail that the individual accumulator is filled again at the beginning of the injection. The hydraulic resistance of the individual accumulator and that of the feed line are coordinated with each other, i.e., the connecting line from the rail to the individual accumulator has a hydraulic resistance that is as high as possible. In a conventional common-rail system without individual accumulators, the hydraulic resistance between the rail and the injector should be as low as possible in order to realize unhindered injection.
DE 195 16 923 A1 also describes a method for controlling an internal combustion engine, in which the pressure level is measured in a line that connects the injection pump and the injection nozzle. The fuel quantity is computed by normalizing the pressure distribution curve and forming the surface integral, with the actual fuel quantity being computed with the use of a constant of proportionality. The method described in the cited document cannot be used in a common-rail system with individual accumulators due to the structural differences. For example, an injection nozzle driven by an injection pump is a passive element, whereas the injector in a common-rail system can be actively driven.